Process for the thermal cracking of catalytic cycle gas oil stocks



Jan. 7, 1958 J. B.l WILLIS ET AL 2,819,203

PROCESS FOR THE THERMAL CRACKING 0F CATALY'IIC CYCLE GAS OIL STOCKS Filed April l5, 1950 tu q Julian B.Willis Thorwell H. Poulsen INVENTORS BY Mw, mux@ ATTORNEYS PROCESS FOR THE THERMAL CRACKING .OF CATALYTIC -CYCLE GAS OIL STOCKS Julian B. Willis, Lansing, lll., and Thorwell H. Paulsen,

Hammond, Ind., assignors to Sinclair Refining Company, New York, N. Y., a corporation of Maine Application April 15, 1950, Serial No. 156,142

1 Claim. (Cl. 196-50) Our invention relates to improvements in thermal cracking, particularly thermal cracking of catalytic cycle stocks, so as to handle light and heavy oil fractions thereof selectively in a single unit. Although emphasis in the petroleum refining has shifted from thermal cracking to catalytic cracking, thermal cracking has retained a substantial place of importance because the cycle stocks, that is the cracked gas oils, produced in large quantities as an incident to catalytic cracking are particularly refractory and are advantageously cracked thermally to obtain increased gasoline yield.

In thermal cracking, the stock to be cracked conventionally is subjected to conditions of cracking temperature and time in a cracking coil or heater and then is passed to an evaporator or tar separator and finally to a fractionating tower. In order to maintain continuous operation, coke or pitch produced in the cracking reaction is continuously removed as tar bottoms from the evaporator and a clean overhead is passed to the fractionator where the recycle stock and usually fresh feed are separated for charging to the cracking heater.

The susceptibility of hydrocarbon stocks to cracking depends as is well known on their molecular weight or boiling range and chemical composition. For this reason light and heavy stocks are ordinarily segregated and selectively cracked in separate cracking heaters. In order to obtain heat economies, however, a common evaporator and a common fractionator are conventionally employed. Thus the cycle oil to be cracked is commonly introduced into a thermal cracking unit by charging to the evaporator or tar separator where it serves as a quench and at the same time is heated by direct heat exchange. Alternative- 1y, the cycle oil charge may be introduced through a oncethrough cracking heater into the tar separator. From the tar separator, the 4fresh feed along with the cracked vapors is passed to the fractionating tower where it is separated into light and heavy fractions. The light stock is withdrawn as a sidestream along with material to be recycled, and the heavy stock is withdrawn as bottoms along with recycling heavy components. The light and heavy stocks are then passed to separate heaters. The eluent 'from each of the heaters is passed to the common evaporator to eliminate the cracked tar product as bottoms. In our studies, we have found that the tars produced from the cracking of the light and heavy oil stocks are different in quality. The light oil cracking produces a tar which is lighter than the tar produced by the heavy oil cracking for the same boiling range. Of course, the tar produced from the common evaporator or tar separator of conventional units is a composite of the tar from both the light oil and heavy oil cracking.

We have found that segregation of the tars produced in the light and heavy oil cracking operations results in surprising improvement in gasoline yield and extent of conversion. For in conventional operation, if the tar separated in the common evaporator has a gravity corresponding to the tar from the heavy oil cracking, a portion of the tar `from the light oil cracking operation is taken United States Patent overhead from the evaporator to the fractionating tower. The light oil tar then will be present in the bottom stream taken from the fractionating tower to the heavy oil cracking heater Where it is highly detrimental causing coke formation and lower heater temperatures. On the other hand, if the composite tar separated from the common evaporator corresponds in gravity to the light oil tar, valuable components from the heavy oil stocks will be retained in the evaporator bottoms and thus will be lost to gasoline production.

According to our invention, light and heavy oil stocks separated in the fractionator of a thermal cracking unit are separately cracked in separate cracking heaters. The ellluent from each of the heaters is passed to separate light and heavy tar separators where tar of the lowest gravity in degrees API permitting clean overhead is withdrawn from `each of the separators. The overhead from the separators is withdrawn to the common fractionator where light and heavy fractions including recycle are separated for charging to the cracking heaters and from which cracked products are recovered. Thus according to our invention each tar separator is operated to produce the heaviest tar bottoms possible without taking tarry constituents in the overhead stream to the fractionating tower. Since no tar is introduced into the fractionating tower, the heavy oil heater feed will be uncontaminated with constituents highly susceptible to coking and no uncrackable recycle constituents will be built up in the heavy oil heater feed. Hence a higher crack per pass in the heavy oil heater results and at the same time maximum recovery ot' gasoline yield from the cycle oil charging stock is made.

Our invention will be further described in connection with the accompanying drawing which illustrates a ow plan incorporating the process in schematic form. In the drawing catalytic cycle oil is introduced through line 1 into light oil tar separator 2 and heavy oil tar separator 3. The cycleoil charging stock picks up heat by direct heat exchange in the evaporators and is then introduced into fractionator 5 through overhead line 4 by vaporization. In the common fractionator the 'light oil fraction of the cycle stock is separated and withdrawn as a side stream through line 6 and is charged through light oil cracking heaters 7 and 8 in parallel and then through lines 9 and 10 to light oil tar separator 2. The light oil tar of lowest API gravity consistent with clean overhead is withdrawn through line 11. The heavy oil fraction of the cycle oil charging stock is withdrawn as bottoms through line 12 and is charged through line l2 to heavy oil cracking heater 13 and thence through line ld to heavy oil tar separator 3 where a heavy oil tar of the lowest API gravity consistent with clean overhead is withdrawn through line 15. The cracked products produced in the process except for tar are taken overhead with the flashed fresh charging stock from each of the tar separators 2 and 3 to the fractionator 5 where gas and gasoline are taken overhead through line 16. Cracked gas oil may be removed as side stream through line 17.

The unexpected extent of improvement effected by segregating the light and heavy oil cracked streams so as to separately withdraw tar lfrom each is illustrated in the following example which compares conventional operation with operation according to our invention. In the conventional method, 8204 barrels per day of catalytic cycle oil were fed partly through a once-through cracking heater and partly into the conventional common tar separator. The fractionator sidestream and bottoms were charged to separate cracking heaters and the heater ellluents were introduced into the common tar separator to reject the tar introduced into the cracking reaction. Gas and gasoline were produced as fractionator overhead and a side stream was also withdrawn.

In operation according to our method, 8204 barrels per day of catalytic cycle stock were fed into the unit as tar separator quench to light and heavy tar separators. The fractionator side stream and bottoms were fed `through separate cracking heaters and the heater ellluents then were passed to the separate light and heavy tar separators. Gas and gasoline were produced as fractionator overhead and a side stream was Withdrawn. The yields produced in each operation are set out in the following table.

Common Separate Evaportor Evaporators Stream BJI). Vol. 13./1). Vol.

percent percent 789 9. 6 994 l2. l 405 4. Q 516 (i. 3 2, 933 35. 8 3, 175 38. 7 72() 8.8 574 7. 0 3, 595 43. 8 3, 192 38. 8 238 -2. 9 -27 -2. 9

handled as separate charge streams. For example, the method is applicable to an operation combining naphtha reforming with cycle oil cracking. The reforming may be performed by charging the naphtha feed once-through to one of the light oil heaters while the gas oil cracking may be performed in parallel in the other light oil heater and the heavy oil heater.

We claim:

In thermal cracking of catalytic cycle gas oil stocks in which light and heavy gas oil fractions of the catalytic cycle charge stock are separated in a fractionator and separately cracked in light and heavy oil cracking heaters, the steps of cracking the light and heavy Oil fractions in scparate cracking heaters, passing the eluent from each of the heaters to separate light and heavy tar separators, preheating the catalytic cycle charge stock by direct heat exchange with the cracking heater efiluent in at least one of the tar separators, withdrawing tar of lowest gravity permitting clean overhead from each of the separators, passing the overhead from the separators to a common rfractionator where the light and heavy fractions arc separated for charging to the cracking heaters and recovering cracked products from the common fractionator.

References Cited in the file of this patent UNITED STATES PATENTS 1,822,753 Smith Sept. 8, 1931 2,175,663 Herthel Oct. lO, 1939 2,188,363 Kuhn Jan. 30, 1940 2,197,009 Pew Apr. 16, 1940 2,335,551 Trow Nov. 30, 1943 2,368,704 Calson Feb. 6, 1945 

